In recent years, the popularity of and the development in optical devices have driven the need for optical glasses having a wide range of characteristics. Especially, a medium-refractive-index low-dispersion optical glass is in great demand because of its usability in a variety of optical devices such as digital cameras and video cameras. Furthermore, due to recent rapid advances in high performance products, there is an increasing demand for a glass suited for precision press molding by which a lens having a complicated shape may be manufactured relatively easily, a highly water-resistant glass that is capable of withstanding a severe use environment and easy to handle, and a glass having a low specific gravity from the perspectives of reducing weight and cost of such a product.
Examples of conventionally known medium-refractive-index low-dispersion optical glass that may be precision press-molded include the one containing Al2O3 in an amount of 4.8% by mass or less (refer to, e.g., Patent Literature 1), the one containing Li2O in an amount of 12.5% by weight or more (refer to, e.g., Patent Literature 2), the one containing CaO in an amount of 18% by weight or more (refer to, e.g., Patent Literature 3), the one containing SrO (refer to, e.g., Patent Literature 4), and the one containing BaO (refer to, e.g., Patent Literature 5).
However, the optical glasses described in Patent Literatures 1 to 5 have not been satisfactory yet as a medium-refractive-index low-dispersion optical glass that may be precision press-molded, in terms of water resistance and specific gravity.